Hello, and thank you for contacting DeviantArt Customer Support. 

For technical reasons, users can watch a maximum of 5,000 deviants, and it appears you’ve reached this limit. Consider visiting the Watching page to seek out deactivated or inactive accounts, which you can then unwatch, freeing up space to watch new deviants in the future.

I hope that resolves your concern, and thank you again for reaching out.